The present invention relates to an electronic process controller having simulated analog control functions.
Present systems for controlling complex industrial processes are relatively inflexible. Programmable logic controllers such as those described in U.S. Pat. No. 3,686,639 issued to W. B. Fletcher et al. and U.S. Pat. No. 3,829,842 issued to D. H. Langdon et al. are useful logic controllers for controlling the relatively straightforward operation of switch-operated machines, but they are of only limited utility in more complex processes where optimal control cannot be predicated on simple on-off decisions.
Control of complex processes requiring analysis of analog as opposed to simple digital signals has typically required the use of hardwired control circuits or specially programmed general purpose computers.
The control circuit approach involves physically interconnecting mechanical or electrical control blocks into the proper configuration for controlling the process in question. Each control block is a separate piece of hardware, such as a circuit board; and the blocks are interconnected into the desired circuit by hard wire. This approach is relatively inflexible in that modification of the process control requires breaking and rewiring of the hard wire connections. In addition, relative to the process control system described herein, the control circuit approach is:
1. expensive in medium to large size systems due to the cost of control blocks; PA1 2. time consuming due to the labor required to mount and interconnect the blocks; PA1 3. limited in accuracy of control; and PA1 4. limited in the variety of control methods available.
An alternative approach to controlling complex processes involves the use of a general purpose computer specifically programmed to control the process in question. See, for example, U.S. Pat. No. 3,761,692 issued to E. G. Cope and U.S. Pat. No. 3,681,782 issued to F. T. Scanlon.
While the use of a specially programmed general purpose computer offers advantages in accuracy and variety of control as compared with the control circuit approach, it nonetheless comprises a relatively inflexible process control system. The programs required for specific process control are typically long, complex, and internally interrelated. They are written in specialized computer languages such as FORTRAN and BASIC or even Assembly Language. As a consequence, control systems using such computers cannot be readily modified by industrial engineers who are not specialists in computer programming.